Systems and methods for producing substitute natural gas

1. A method for producing a synthetic gas, comprising:

gasifying a feedstock within a gasifier to provide a raw syngas;

processing the raw syngas within a purification system to provide a treated syngas, wherein the purification system comprises a flash gas separator;

converting the treated syngas and a first heat transfer medium into a synthetic gas, a second heat transfer medium, and a methanation condensate; and

introducing the methanation condensate to the flash gas separator;

wherein converting the treated syngas comprises:

splitting the treated syngas into a first treated syngas, a second treated syngas, and a third treated syngas;

converting the first treated syngas into a first effluent in a first methanator; mixing the first effluent and the second treated syngas to provide a first mixed effluent; converting the first mixed effluent into a second effluent in a second methanator;

mixing the second effluent and the third treated syngas to provide a second mixed effluent; and

converting the second mixed effluent into a third effluent in a third methanator;

removing a first condensate from the third effluent in a first separator to provide a first separated effluent; and

converting the first separated effluent into a fourth effluent in a fourth methanator.

2. The method of claim 1 , further comprising transferring heat from the fourth effluent to the first heat transfer medium to provide a cooled effluent.

3. The method of claim 2 , further comprising removing a second condensate from the cooled effluent to provide a second separated effluent.

4. The method of claim 3 , further comprising compressing the second separated effluent to provide a compressed effluent.

5. The method of claim 4 , further comprising removing a third condensate from the compressed effluent to provide the synthetic gas.

6. The method of claim 5 , wherein at least one of the first, second, and third condensates at least partially comprises at least a portion of the methanation condensate.

7. A method for producing a synthetic gas, comprising:

gasifying a carbonaceous feedstock in the presence of an oxidant within a gasifier to provide a raw syngas;

cooling the raw syngas within a cooler to provide a cooled syngas;

processing the cooled syngas within a purification system to provide a treated syngas, wherein the purification system comprises a flash gas separator and a saturator;

introducing the treated syngas and a first heat transfer medium to a methanator to provide a synthetic gas, a second heat transfer medium, and a first condensate;

introducing the first condensate to the flash gas separator to provide a flashed gas and a second condensate;

introducing the flashed gas to the gasifier; and

introducing the second condensate to the saturator.

8. The method of claim 7 , wherein processing the cooled syngas within the purification system further comprises increasing a moisture content of at least a portion of the cooled syngas with the saturator to provide a saturated syngas.

9. The method of claim 8 , further comprising:

introducing the saturated syngas to a gas shift device to provide a shifted syngas;

introducing the shifted syngas to a syngas cooler to provided a cooled shifted syngas and a third condensate; and

introducing the third condensate to the flash gas separator.

10. The method of claim 7 , wherein processing the cooled syngas within the purification system further comprises:

introducing at least a portion of the cooled syngas to a hydrolysis device to provide a hydrogen sulfide syngas; and

removing ammonia from the hydrogen sulfide syngas with an ammonia scrubber to provide a scrubbed syngas and waste water.

11. The method of claim 10 , further comprising:

introducing the waste water to a syngas cooler to provide a third condensate; and

introducing the third condensate to the flash gas separator.