Abstract: A method is described for recovering olefins by condensation from the reaction effluent stream from a cracking furnace containing olefins and hydrogen wherein a membrane separator is employed to reject the hydrogen from the reaction effluent stream. In such a manner, the dew point temperature of the stream can be increased and the energy required to provide refrigeration for olefin condensation can be reduced. The method comprises primary condensation, vapor-liquid separation and membrane hydrogen rejection steps followed by a series of cascaded chilling and vapor-liquid separation steps. Condensate recovered is processed in a demethanizer to separate light end components. Also disclosed is an olefins plant employing a membrane separator for rejecting hydrogen to enhance the energy efficiency and/or facilitate the expansion of plant capacity.