This invention is directed to methods of removing water and other condensable materials, as well as solids particles such as catalyst particles, from olefin product streams so as to reduce fouling in the liquid and vapor separation equipment. In order to reduce fouling or contamination in the condensing or quenching process, this invention includes adding a hydrocarbon to at least a portion of the condensed liquid fraction in an amount that effects separation of the liquid fraction into upper and lower fractions.