NOVELTY - The method comprises separating portions of carbon dioxide (CO2) and hydrogen sulfide (H2S) from a portion of a syngas stream, channeling the separated CO2 and H2S portions to a sequestration system or a gasification 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 CO2.    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 efficiently and 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 portions of carbon dioxide (CO2) and hydrogen sulfide (H2S) from a portion of a syngas stream, channeling the separated CO2 and H2S portions to a sequestration system or a gasification 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 CO2 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 CO2 and H2S portions from the syngas stream portion comprises: channeling the shifted syngas stream including the CO2 and H2S to an acid gas removal unit (AGRU); and separating the portions of the CO2 and H2S from the portion of the shifted syngas stream within the AGRU by forming a CO2 stream containing H2S below or above a predetermined limit to form a H2S-lean CO2 stream or forming a H2S acid gas stream. The CO2 stream is formed by injecting a portion of the H2S-lean CO2 stream into the gasification reactor or the sequestration system. INDEPENDENT CLAIMS are included for:    (1) a gasification system; and    (2) an integrated gasification combined-cycle power generation plant.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic diagram of a gasification system of an integrated gasification combined-cycle power generation plant.    Reactors (208, 212)    Acid gas removal unit (218)    Gasification system (300)    Cooled methanation reactor (302)    Heat transfer apparatus. (306)