NOVELTY - Heat exchanger (90) comprises a thermally conductive substrate having two faces. A first channel is formed on the substrate first face. It circulates steam discharged from a fuel cell stack (10) of a fuel cell system (100). A second channel is formed on the substrate second face. It circulates water and supplies the water to a reformer (30) of the fuel cell system. A first cover plate is disposed on the substrate first face. A second cover plate is disposed on the substrate second face.    USE - For use in a fuel cell system useful for generating electrical energy.    ADVANTAGE - The heat exchanger is able to utilize the heat energy of the byproduct discharged from the fuel cell stack or the reformer to pre-heat the water that is to be supplied to the reformer thus the overall heat efficiency and performance of the reformer are enhanced. It does not require a case and ventilation fan to condense the steam produced in the fuel cell stack. The overall size of the fuel cell system is made more compact. The parasitic electrical energy required to drive the ventilation fan is not necessary, thus no noise is generated from the ventilation fan. The heat exchanger improves overall performance and energy efficiency of the fuel cell system.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a fuel cell system comprising the fuel cell stack for producing electricity by a reaction of hydrogen and oxygen. The reformer is coupled to the fuel cell stack for reforming fuel. It produces a reformed gas and supplies the reformed gas to the fuel cell stack. A fuel supply source (50) is coupled to the reformer. It supplies a fuel to the reformer. A water supply source (70) stores and supplies water to the reformer. The heat exchanger is coupled to each of the fuel cell stack, the reformer, and the water supply source. The first cover plate contacts the first face and covers the first channel to form a first passageway in the first channel. The second cover plate contacts the second face and covers the second channel to form a second passageway in the second channel.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of a fuel cell system.    Fuel cell stack (10)    Reformer (30)    Fuel supply source (50)    Water supply source (70)    Heat exchanger (90)    Fuel cell system (100)