<p>Disclosed herein is a method for separating from the reactor effluent of an olefin oligomerization procedure those catalyst materials and polymeric by-products which can cause difficulties in the downstream processing of such effluent. Polymer by-products and catalyst in the effluent are separated from reaction products by flash vaporization utilizing an in-situ hot solvent which is contacted with the effluent and serves as the heating medium to promote this flash vaporization step. Subsequent processing of a liquid portion of the effluent which is left after flash vaporization involves recovery of catalyst and polymeric by¬ products therefrom in a steam stripping vessel. Also disclosed is a multiple reactor system which can be used for continuous trimerization of ethylene to 1-hexene while at the same time washing polymeric by-products from one of the reactors in the series using a wash oil solvent.</p>