The invention relates to a method for using reaction heat produced by reaction during the production of 1,2-dichloroethane from ethylene and chlorine in a direct chlorination reactor. The chlorine is produced in a sodium chloride electrolysis and the reaction heat, during the formation of 1,2-dichloroethane is used at least partially for the evaporation of NaOH, which is produced during NaCl-electrolysis for producing the required chlorine for direct chlorination, as a coupling product. The invention also relates to a device for carrying out said method, comprising a multi-tube heat exchanger comprising two fixed tubular plates and a NaOH-liquid phase part, and the caustic soda passes through the inside of the tube and 1,2-dichloroethane passes the outside of the tube. The heat exchanger also comprises devices for feeding and distributing the caustic soda in the inside of the tube.