The invention relates to a gas- and steam turbine unit (1) comprising a steam generator (30) which uses waste heat and is mounted downstream from the gas turbine (2) on the flue gas side. The heating surfaces of the generator are mounted in the water-steam circuit (24) of said steam turbine (20). A gasification device (132) is mounted downstream from the gas turbine (2) through a fuel duct (130) for integrated gasification of a fossil fuel (B). A saturator (150) is mounted in the fuel duct (130). Reliable operation of said saturator (150) is required for such a gas- and steam turbine unit (1) irrespective of the operation mode of said gasification device (132). In addition, a steam water-heat exchanger (184), mounted on the secondary side of said saturator circuit (132), is fed on its primary side with feeding water (S) drawn from the water-steam circuit (24) of the steam turbine (20). The feed water (S) which is cooled in the water-heat exchanger of the saturator (184) can be heated by means of a partial stream (T) of compressed air, whereby said partial stream (T) of compressed air can transferred to an air separation facility (138) mounted upstream from said gasification device.