NOVELTY - The method comprises separating a portion of carbon dioxide from a portion of a syngas stream, channeling the separated portion of the carbon dioxide to a portion of a gasification reactor, producing a substitute natural gas (SNG) stream from a portion of a shifted syngas stream within a methanation reactor, channeling the shifted syngas stream portion to the methanation reactor from an acid gas removal unit (AGRU) or a gas shift reactor, and coupling a portion of a steam generation system in heat transfer communication with a portion of the gas shift reactor or the methanation reactor.    USE - The method is useful for producing substitute natural gas for an integrated gasification combined-cycle power generation plant.    ADVANTAGE - The method is capable of optimally generating and collecting heat from the exothermic chemical reactions thus improving the thermal efficiency of the integrated gasification combined-cycle power generation plant, and reduces the nitrous oxides emission without requiring diluent addition thus reducing the capital and labor costs of the plant.    DETAILED DESCRIPTION - The method comprises separating a portion of carbon dioxide from a portion of a syngas stream, channeling the separated portion of the carbon dioxide to a portion of a gasification reactor, producing a substitute natural gas (SNG) stream from a portion of a shifted syngas stream within a methanation reactor, channeling the shifted syngas stream portion to the methanation reactor from an acid gas removal unit (AGRU) or a gas shift reactor, and coupling a portion of a steam generation system in heat transfer communication with a portion of the gas shift reactor or the methanation reactor. The syngas stream is provided by producing the syngas stream with the gasification reactor, channeling a portion of the syngas stream to the gas shift reactor, and producing the shifted syngas stream including the carbon dioxide in the gas shift reactor. The shifted syngas stream is produced by transferring heat from the portion of the gas shift reactor via a heat transfer apparatus. The separating step of the carbon dioxide portion comprises channeling the shifted syngas stream to the AGRU, and separating the carbon dioxide portion from the portion of the shifted syngas stream within the AGRU. The channeling step of the carbon dioxide portion comprises forming a carbon dioxide stream, and injecting a portion of the carbon dioxide stream into the gasification reactor. The SNG stream is produced by transferring heat from a portion of the methanation reactor via the heat transfer apparatus. INDEPENDENT CLAIMS are included for:    (1) a gasification system; and    (2) an integrated gasification combined-cycle (IGCC) power generation plant.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic diagram of an integrated gasification combined-cycle (IGCC) power generation plant.    Gas turbine engine (110)    Compressor (112)    Turbine (114)    Steam turbine engine (130)    Steam generation system (140)    Heat transfer apparatus. (144)