The oligomerization of ethylene using a chromium catalyst having a heteroatomic ligand is used to provide oligomerization products that are selective towards hexene and/or octene. However, such processes also typically produce some polymer as an undesired by-product. The present invention is directed towards improvements in the selective oligomerization of ethylene wherein said process makes use of heat exchangers to provide heat where polymer by-product fouling of the reactor may occur.