NOVELTY - The fuel cell system comprises water transfer device comprising a device cathode effluent input connected to the cathode output, device supply stream output connected to either or both of the fuel cell inputs, and a water-transfer membrane. The water transfer device transfers water from the cathode effluent to either or both of the supply streams.    USE - For use in producing electricity from hydrogen and oxidant.    ADVANTAGE - The inventive system includes water transfer devices that can afford advantages over water management systems known in the prior art. It is capable of maintaining an overall water balance in the system under a range of operating conditions, reduced energy requirements, reduced component complexity, and reliability, and enhanced operating safety.    DETAILED DESCRIPTION - A fuel cell system comprises fuel cell (11), and water transfer device (10). The fuel cell comprises an anode input for a hydrogen-containing anode supply stream, a cathode input for an oxidant-containing cathode supply stream, an anode effluent output, and a cathode output for cathode effluent comprising water produced by the fuel cell. The water transfer device comprises a device cathode effluent input connected to the cathode output, device supply stream output connected to either or both of the fuel cell inputs, and a water-transfer membrane. The water transfer device transfers water from the cathode effluent to either or both of the supply streams, and where the temperature of the cathode effluent at the device cathode effluent input is not significantly greater than the temperature of the cathode effluent at the cathode output. The temperature at the device input being sufficient to maintain water in its vapor state and being greater than its dew point and less than or equal to 10 degrees C above its dew point.    An INDEPENDENT CLAIM is also included for a method of operating a fuel cell power plant comprising transferring water from the reformate supply stream to a reactant stream in a first water transfer device that has a high pressure side and a low pressure side, by transporting the one or both of the reactant streams through the low pressure side, and by transporting the reformate supply stream through the high pressure side; and transferring water from one or more of the reaction oxidant stream the reformate supply stream, and the cathode oxidant supply stream in a second water transfer device that has a high pressure side and a low pressure side, by transporting the reactor oxidant stream, the reformate supply stream, and the cathode oxidant supply stream though the low pressure side, and by transporting one or both of the effluent streams (12) through the high pressure side.    DESCRIPTION OF DRAWING(S) - The drawing shows a diagram depicting the inventive fuel cell system.    Transfer device (10)    Fuel cell (11)    Effluent stream (12)    Input stream (13)    Combustor (14)