A method involved operating a combined-cycle power plant (10), which has a gas turbine (11) with a compressor (12) and a turbine (13), a heat recovery steam generator (17) which is connected downstream to the gas turbine (11) and is for producing steam in a water/steam cycle, and also at least one once-through cooler (21), through which flows compressed air which is compressed in the compressor (12) and intended for cooling the gas turbine (11), and, cooling down, converts feed-water (24) which is fed from the heat recovery steam generator (17) into steam, and discharges the steam to the heat recovery steam generator (17). Making the operation more flexible is achieved, without lowering the load of the gas turbine (11), by the combined-cycle power plant (10) being switched between a first operating mode, in which only the gas turbine cycle is used for power generation, and a second operating mode, in which the gas turbine cycle and the water/steam cycle are used for power generation.