Abstract: A waste-heat steam generator of a gas-turbine and steam-turbine system. The steam-generating heat content in the expanded working medium from the gas turbine is used for the steam turbine connected into a water/steam loop. Condensed steam from the steam turbine is supplied as condensate to the water/steam loop. A partial flow is diverted from the water-steam loop for selectively preheating the condensate or the fuel with the partial flow. To achieve high efficiency, irrespective of the fuel used for the gas turbine, when oil is used as the fuel for the gas turbine, the partial flow from the water/steam loop is used for condensate preheating. When gas is used as the fuel for the gas turbine, the partial flow is used for fuel preheating. A heat exchanger for the selective preheating either of the gas-turbine fuel or of the condensate with the partial flow is provided.