NOVELTY - Cooling an exothermic reaction zone includes introducing water stream and hydrocarbon-containing stream into humidifying tubes extending through catalytic exothermic reaction zone, introducing process stream into reaction zone, humidifying hydrocarbon-containing stream with water in humidifying tubes, withdrawing cooled reaction product of exothermic reaction from reaction zone and withdrawing hydrocarbon-containing stream from humidifying tubes.    USE - For cooling an exothermic reaction zone useful in fuel processing system for fuel cells.    ADVANTAGE - The inventive process has reduced investment cost and improved process economics.    DETAILED DESCRIPTION - Cooling an exothermic reaction zone includes introducing water stream and hydrocarbon-containing stream into humidifying tubes extending through a catalytic exothermic reaction zone of a catalytic fixed bed with solid catalyst; introducing a process stream into the reaction zone for catalytic exothermic reaction(s); passing the water stream in a falling film along the inner circumference of humidifying tubes; humidifying the hydrocarbon-containing stream with water in the humidifying tubes in indirect heat exchange with the exothermic reaction zone; withdrawing cooled reaction product of exothermic reaction from the reaction zone; withdrawing the heated humidified, hydrocarbon-containing stream from the humidifying tubes; and transferring the heated humidified, hydrocarbon-containing process stream for further processing. An INDEPENDENT CLAIM is also included for a reactor unit for carrying out the above cooling process, comprising a catalytic exothermic reaction zone (5) within a reactor shell. The reactor has an inlet (2) for a hydrocarbon-containing stream, an inlet (3) for a water stream, and an outlet (8) downstream of the reaction zone. Each inlet is placed upstream of the catalytic exothermic reaction zone. The catalytic exothermic reaction zone comprises an inlet (6) for a process stream, an outlet (7) for the process stream reaction product, a catalytic fixed bed with solid catalyst, and humidifying tubes (4) for humidifying the hydrocarbon-containing stream. The humidifying tubes extend throughout the exothermic reaction zone and are open at either end. The tubes are adapted to create a falling film of water along their inner circumference to exchange heat by indirect heat contact with the reaction zone.    DESCRIPTION OF DRAWING(S) - The figure is a schematic diagram of a reactor unit.    Reactor (1)    Inlets (2, 3, 6)    Humidifying tubes (4)    Reaction zone (5)    Outlets (7, 8, 11)    De-mister (9)    Water reservoir (10)