NOVELTY - Carbon monoxide (CO) is oxidized in a preferential oxidizer for a hydrogen reformer of a fuel cell system by introducing a hydrogen-rich reformate to heat exchange system with a single integrated oxidation reaction region having oxidation catalyst; injecting oxygen to hydrogen-rich reformate to oxidize CO and maintaining reformate at predetermined temperature when CO is oxidized.    USE - Oxidizing carbon monoxide to carbon dioxide in a hydrogen reformer of a fuel cell system for stationary and vehicular applications.    ADVANTAGE - The method eliminates the need for stages comprising catalyst beds and heat exchangers for oxidizing carbon monoxide to carbon dioxide, while maintaining a predetermined temperature of the reformate.    DETAILED DESCRIPTION - Oxidation of carbon monoxide in a preferential oxidizer for a hydrogen reformer of a fuel cell system, comprises:    (a) providing a heat exchange system (23) of the preferential oxidizer, having a single integrated oxidation reaction region (31) in which an oxidation catalyst is disposed for oxidizing carbon monoxide to carbon dioxide;    (b) introducing a hydrogen-rich reformate to the heat exchange system, at a predetermined temperature and having a first content of carbon monoxide;    (c) injecting oxygen to the hydrogen-rich reformate, at a predetermined stoichiometric oxygen/carbon monoxide ratio;    (d) oxidizing the carbon monoxide to carbon dioxide with the oxidation catalyst when the hydrogen-rich reformate and oxygen contact the single integrated oxidation reaction region so that the hydrogen-rich reformate has a second content of carbon monoxide less than the first content at the predetermined temperature; and    (e) maintaining the hydrogen-rich reformate at the predetermined temperature when the carbon monoxide is oxidized in the single integrated reaction region of the heat exchange system.    INDEPENDENT CLAIMS are also included for:    (1) an integrated catalyst-heat exchanger for a hydrogen reforming process of a fuel cell system, comprising tubes having an inlet, an outlet for coolant, and an external surface; fin(s) disposed on the external surface and extending along the external surface, having a contact area; and an oxidation catalyst applied on the contact area of the fin for oxidation of carbon monoxide; and    (2) a method of making an integrated catalyst-heat exchanger for a hydrogen reforming process of a vehicle fuel cell system, comprising providing a heat exchanger for the hydrogen reforming process, including tube(s) as above; holding the heat exchanger in a fixture; applying an oxidation catalyst on the contact area of the fins for carbon monoxide oxidation during the hydrogen reforming process; and drying the oxidation catalyst applied on the contact area of the fins.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional side view of the preferential oxidation reactor depicting a heat exchanger system.    Preferential oxidation reactor (10)    Heat exchange system (23)    First heat exchanger (28)    Integrated catalyst heat exchanger (30)    Integrated oxidation reaction region (31)