Abstract: An improved olefin upgrading technique and fixed-bed reactor system has been developed for increasing production of premium heavy hydrocarbons, such as distillate fuel, from lower olefinic feedstock. During recovery and recycle of intermediate range hydrocarbons products, a technique has been found for withdrawing a fraction rich in C.sub.5 -C.sub.9 gasoline range olefinic hydrocarbons from the oligomerization reactor effluent stream. By separating the reaction effluent in a multi-stage distillation system, fractionation feed can be separated into a heavier bottoms stream rich in C.sub.10 + hydrocarbons and a light hydrocarbon overhead, while withdrawing a liquid stream as an overflash fractionation stream rich in intermediate hydrocarbons. The overflash stream is combined to form a portion of the recycle stream to the reactor, thereby providing a more efficient and lower cost recovery process.