NOVELTY - A water-gas shift (WGS) reactor system, comprises a first stage WGS reactor receiving a reformate gas; a heat exchanger (20, 24, 82) receiving the reformate gas from first stage reactor and cooling the reformate gas, and including a catalyst that converts carbon monoxide and water to carbon dioxide and hydrogen; and a second stage WGS reactor receiving the cooled reformate gas.    USE - The WGS reactor system is for fuel processing system used for producing hydrogen useful to produce electricity for a fuel cell (claimed). The fuel cell is used in automotive industry as source of power for vehicles.    ADVANTAGE - The WGS reactor system minimizes the mass and volume of the fuel processing system, especially if the system is being used directly on the vehicle, thus satisfying vehicle weight and size requirements. The fuel processing system has a high power density, which is the ratio of output power to volume of the system. The size of the components in the system is reduced.    DETAILED DESCRIPTION - A WGS reactor system, comprises:    (a) a first stage WGS reactor receiving a reformate gas, and including a catalyst that converts carbon monoxide and water to carbon dioxide and hydrogen;    (b) a heat exchanger receiving the reformate gas from the first stage reactor and cooling the reformate gas, and including a catalyst that converts carbon monoxide and water to carbon dioxide and hydrogen; and    (c) a second stage WGS reactor receiving the cooled reformate gas from the heat exchanger, and including a catalyst that converts carbon monoxide and water to carbon dioxide and hydrogen.    INDEPENDENT CLAIMS are also included for:    (a) a heat exchanger for cooling a main fluid, comprising an inlet for receiving a cooling fluid; an internal structure through which the cooling fluid and the main fluid propagate, the structure including a catalyst that converts carbon monoxide to carbon dioxide in the main fluid; and an outlet through which heated cooling fluid exits the heat exchanger;    (b) a fuel processing system (10) for producing a hydrogen reformate gas, comprising a primary reactor (14) receiving a liquid hydrocarbon fuel and generating a reformate gas including hydrogen and carbon monoxide; a first heat exchanger receiving the reformate gas from the primary reactor and cooling the reformate gas; a WGS reactor assembly including a first stage WGS reactor receiving the cooled reformate gas from the first heat exchanger, a second heat exchanger receiving the reformate gas from the first stage reactor and cooling the reformate gas, and a second stage WGS reactor receiving the cooled reformate gas from the second heat exchanger, each of the first stage reactor, second heat exchanger and second stage reactor including a catalyst that converts carbon monoxide and water to carbon dioxide and hydrogen; a third heat exchanger receiving the reformate gas from the second stage reactor and cooling the reformate gas; and a preferential oxidation reactor receiving the cooled reformate gas from the third heat exchanger, and including a catalyst that selectively oxidizes carbon monoxide to carbon dioxide in the reformate gas; and    (c) a method of cooling a reformate gas in a fuel processing system, comprising providing a heat exchanger including a catalyst that converts carbon monoxide and water to carbon dioxide and hydrogen by an exothermic reaction; and causing the reformate gas to flow through the heat exchanger to be cooled.    DESCRIPTION OF DRAWING(S) - The figures show a schematic plan view of a fuel processor systems, and a schematic plan view of WGS reactor assembly for the fuel processing system.    Fuel processing system (10)    Primary reactor (14)    Heat exchanger (20, 24, 82)