Polyolefins, especially branched polyethylenes, can be made by combining a catalyst that can oligomerize ethylene to one or more alpha-olefins and a polymerization catalyst which can copolymerize ethylene and null-olefins, by first carrying out the process in, for example, a continuous stirred tank reactor or its equivalent, and then in a plug flow reactor or its equivalent. The process minimizes the occurrence of (unreacted) null-olefins in the final polymer product.