NOVELTY - A reactor assembly comprises two water gas shift (WGS) reactors and a heat exchanger, each containing a catalyst for converting carbon monoxide and water to carbon dioxide and hydrogen. The first stage (WGS) reactor receives a reformate gas and passes the same to the heat exchanger for cooling reformate gas and the cooled reformate gas is passed on to the second stage (WGS) reactor.    USE - In fuel processing system, where the (WGS) reactor system is positioned between the primary reactor and the preferential oxidation reactor, to produce hydrogen for fuel cell (claimed).    ADVANTAGE - By providing the catalyst within the heat exchanger in the (WGS) reactor assembly, the assembly becomes smaller than what is currently known in the art. Because the reactors can be made smaller, the size of the fuel processor system can be made smaller, which has a direct impact on the vehicle requirements. Reduced start-up time of the fuel processor system reduces the thermal mass of the (WGS) reactor assembly. If the hot gas from the combustor can be introduced into the heat exchanger during start-up, this would warm up the (WGS) reactor assembly and make the fuel processor system start producing hydrogen more quickly. Because of the temperature gradient from the first stage reactor output temperature of 370degreesC down to the second stage reactor input temperature of 300 - 310degreesC reduces the amount of the catalyst needed. For this temperature gradient, it would enhance shift reaction kinetics and reduce the use of the catalyst.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a heat exchanger for cooling a main fluid (preferably reformate gas) comprising an inlet for receiving a cooling fluid, an internal structure, containing the catalyst that converts carbon monoxide to carbon dioxide, for passage of the cooling and main fluids and an outlet through which heated coolant exits the heat exchanger;    (2) a fuel processing system for producing a hydrogen reformate gas comprising a primary reactor for receiving a liquid hydrocarbon fuel and generating the reformate gas of hydrogen and carbon monoxide, a first heat exchanger for receiving and cooling the reformate gas from the primary reactor, the (WGS) reactor assembly containing a second heat exchanger for cooling the reformate gas, a third heat exchanger for receiving and cooling the reformate gas from the second stage reactor of the (WGS) reactor assembly and a preferential oxidation reactor containing the catalyst for receiving cooled reformate gas from the third heat exchanger and selectively oxidize carbon monoxide to carbon dioxide in the reformate gas; and    (3) cooling reformate gas in fuel processing system by passing the reformate gas through heat exchanger containing the catalyst.