A method is disclosed for treating the effluent from a hydrocarbon pyrolysis process unit to recover heat and remove tar therefrom. The method comprises passing the gaseous effluent to at least one primary heat exchanger, thereby cooling the gaseous effluent and generating high pressure steam. Thereafter, the gaseous effluent is passed through at least one secondary heat exchanger having a heat exchange surface maintained at a temperature such that part of the gaseous effluent condenses to form in situ a liquid coating on said surface, thereby further cooling the remainder of the gaseous effluent to a temperature at which tar, formed by the pyrolysis process, condenses. The condensed tar is then removed from the gaseous effluent in at least one knock-out drum.