The present invention relates to an in-line method for generating comonomer, such as 1-hexene or 1-octene, from monomer, such as ethylene. The comonomer generated is directly transported, without isolation or storage, to a polyethylene polymerization reactor. The in-line method for generating comonomer includes the steps of providing an in-line comonomer synthesis reactor and a downstream gas/liquid phase separator prior to a polyethylene polymerization reactor; feeding ethylene monomer and a catalyst in a solvent to the comonomer synthesis reactor; reacting the ethylene monomer and the catalyst in solvent under reaction conditions to produce an effluent stream including ethylene monomer and comonomer; passing the effluent stream from the comonomer synthesis reactor to the downstream gas/liquid phase separator to separate a gas stream from a bottom stream, wherein the gas stream is a mixture of ethylene monomer, and comonomer; and passing the gas stream to the polyethylene polymerization reactor to provide the necessary comonomer input. The in-line method is useful in the production of LLDPE, and other branched polyethylene based polymers. Some benefits include process simplification and reduced capital and operating costs.