To increase turbine output by disposing a water heating unit for saturating fuel gas in parallel with a low-pressure economizer of a heat recovery steam generator and thereby control a temperature difference between a high- temperature heat exchange stream and a low-temperature heat exchange stream to a level or lower level of a temperature of a steam generator that has the lowest pressure.

In a combined cycle system where steam is generated at a heat recovery steam generator 132 utilizing exhaust gas from a gas turbine 112 and a steam turbine 118 is driven by the steam, a fuel gas saturation unit 160 having a water inlet, fuel gas inlet, saturated fuel gas outlet, and water outlet is provided. Moisture in fuel gas supplied to the saturation unit 160 is absorbed by letting the fuel gas in contact with hot water in a packed tower and the like. Water at the bottom of the saturation unit 160 is heated by a saturation unit heater 162 disposed in the heat recovery steam generator 132, and the saturated fuel gas passing through the fuel gas saturation unit 160 is heated further in a fuel heater 164 and supplied to a combustor 114 of the gas turbine.