NOVELTY - A fuel cell system includes fuel cell stack comprising polymer electrolyte membrane (PEM) fuel cells; and fuel processing reactor having inlet and outlet. The inlet and outlet are in fluid communication with a catalyst for converting a hydrocarbon into a gas containing hydrogen and carbon monoxide. The outlet is in fluid communication with anode chamber of fuel cell and the outlet of reactor is in fluid communication with cathode outlet.    USE - For use in converting chemical energy into electrical energy.    ADVANTAGE - The cathode exhaust is flowed into fuel processor via a conduit that is insulated to minimize heat loss from the cathode exhaust along with any associated water loss from condensation. Since the fuel cell stack is tolerant to carbon monoxide, an advantage of the invention is that it is not necessary to include a ruthenium-based catalyst or platinum ruthenium alloy to improve carbon monoxide tolerance, as is common in the prior art, thus the cost associated with the fuel cell stack is reduced.    DETAILED DESCRIPTION - A fuel cell system, comprises fuel cell stack comprising PEM fuel cells each having an operating temperature of greater than or equal to120degreesC, the fuel cell stack having a cathode inlet to receive a flow of ambient air and a cathode outlet to provide a cathode exhaust flow; and fuel processing reactor having an inlet and an outlet. The inlet and outlet are in fluid communication with a catalyst suitable for converting a hydrocarbon into a gas containing hydrogen and carbon monoxide, the outlet being in fluid communication with an anode chamber of the fuel cell and the outlet of the reactor being in fluid communication with the cathode outlet.    An INDEPENDENT CLAIM is also included for a method of operating a fuel cell system, comprising providing a flow of ambient air to a cathode chamber of a PEM fuel cell; reacting a portion of the air in the fuel cell to produce electricity; exhausting a remaining portion of the air from the fuel cell, where the remaining portion of air contains water vapor; mixing a portion of the exhausted air with a hydrocarbon gas to form a feed mixture; providing the feed mixture to a reactor; and routing a fuel flow from the reactor to an anode chamber of the fuel cell.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of an integrated high temperature PEM fuel cell system.    De-sulfurization (604)    Autothermal reactor (608)    Shift reactor (610)    Fuel cell stack (614)    Oxidizer (616)