A process for polymerizing or oligomerizing a hydrocarbon includes feeding a liquid hydrocarbon reactant and a liquid evaporative cooling medium into a bulk liquid phase which includes polymeric or oligomeric product admixed with a catalyst, and allowing at least a portion of the liquid hydrocarbon reactant and the liquid evaporative cooling medium to vaporize to form bubbles rising through the bulk liquid phase, with the hydrocarbon reactant polymerizing or oligomerizing to form the polymeric or oligomeric product and with the evaporation of both the liquid hydrocarbon reactant and the liquid evaporative cooling medium effecting heat removal from the bulk liquid phase. Gaseous components are withdrawn from a head space, cooled and separated. Condensed hydrocarbon reactant and condensed cooling medium are recycled to the bulk liquid phase.