A combined power generation system, comprising: a natural gas synthesis apparatus (100, 1100, 2100, 3100, 4100) comprising a gasifier (110, 1110, 2110, 3110, 4110) for generating a synthetic gas from coal and oxygen and a methanation reactor (160, 1160, 2160, 3160, 4160) for synthesizing methane using the synthetic gas; and a fuel cell apparatus (200, 1200, 2200, 3200, 4200) comprising a cathode electrode (230, 1230, 2230, 3230, 4230), an anode electrode (220,1220, 2220, 3220, 4220), and a reformer unit (210, 1210, 2210, 3210, 4210), wherein fuel comprising the methane generated by the methanation reactor (160, 1160, 2160, 3160, 4160) is supplied to the reformer unit (210, 1210, 2210, 3210, 4210), wherein the natural gas synthesis apparatus (100, 1100, 2100, 3100, 4100) further comprises: a first reactor (140, 1140, 2140, 3140, 4140) for generating hydrogen and carbon dioxide through a water gas shift reaction from a fluid comprising hydrogen and carbon monoxide discharged by the gasifier (110, 4110); and a second reactor (150, 1150, 2150, 3250, 4150) for separating carbon dioxide and sulfur compounds from a fluid that comprises hydrogen and carbon dioxide and that is discharged by the first reactor (140, 1140, 2140, 3140, 4140), through physical or chemical segregation, wherein the carbon dioxide separated by the second reactor (150, 1150, 2150) is supplied to the cathode electrode (230, 1230, 2230), wherein a fluid that comprises hydrogen and carbon monoxide and that is discharged by the anode electrode (1220, 3220, 4220) is introduced into the first reactor (1140, 3140, 4140). a natural gas synthesis apparatus (100, 1100, 2100, 3100, 4100) comprising a gasifier (110, 1110, 2110, 3110, 4110) for generating a synthetic gas from coal and oxygen and a methanation reactor (160, 1160, 2160, 3160, 4160) for synthesizing methane using the synthetic gas; and a fuel cell apparatus (200, 1200, 2200, 3200, 4200) comprising a cathode electrode (230, 1230, 2230, 3230, 4230), an anode electrode (220,1220, 2220, 3220, 4220), and a reformer unit (210, 1210, 2210, 3210, 4210), wherein fuel comprising the methane generated by the methanation reactor (160, 1160, 2160, 3160, 4160) is supplied to the reformer unit (210, 1210, 2210, 3210, 4210), wherein the natural gas synthesis apparatus (100, 1100, 2100, 3100, 4100) further comprises: a first reactor (140, 1140, 2140, 3140, 4140) for generating hydrogen and carbon dioxide through a water gas shift reaction from a fluid comprising hydrogen and carbon monoxide discharged by the gasifier (110, 4110); and a second reactor (150, 1150, 2150, 3250, 4150) for separating carbon dioxide and sulfur compounds from a fluid that comprises hydrogen and carbon dioxide and that is discharged by the first reactor (140, 1140, 2140, 3140, 4140), through physical or chemical segregation, wherein the carbon dioxide separated by the second reactor (150, 1150, 2150) is supplied to the cathode electrode (230, 1230, 2230), a first reactor (140, 1140, 2140, 3140, 4140) for generating hydrogen and carbon dioxide through a water gas shift reaction from a fluid comprising hydrogen and carbon monoxide discharged by the gasifier (110, 4110); and a second reactor (150, 1150, 2150, 3250, 4150) for separating carbon dioxide and sulfur compounds from a fluid that comprises hydrogen and carbon dioxide and that is discharged by the first reactor (140, 1140, 2140, 3140, 4140), through physical or chemical segregation, wherein the carbon dioxide separated by the second reactor (150, 1150, 2150) is supplied to the cathode electrode (230, 1230, 2230), wherein a fluid that comprises hydrogen and carbon monoxide and that is discharged by the anode electrode (1220, 3220, 4220) is introduced into the first reactor (1140, 3140, 4140).