Process for cooling an exothermic reaction zone by introducing a stream of water and a hydrocarbon-containing stream into a plurality of 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 one or more catalytic exothermic reactions, passing the stream of water in a falling film along the inner circumference of the 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 the 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.