The present invention relates to a process for optimizing the separation by distillation of a hydrocarbon-containing feed stream comprising olefin monomer, co-monomer and hydrocarbon diluent. A preferred component stream separated comprises ethylene, 1-hexene and isobutane. A first distillation column (2) realizes a rough cut between a mixture of isobutane, hexene and the heavies (5). The heavy bottom product is treated in a second distillation column (4) and separated into isobutane vapor (6), a purified liquid hexene stream (7) and the heavy components (8). The remaining monomer, isobutane, which exits from the top (9) of the first distillation column (2), is sent to a third distillation column (3) to be sepatated into isobutane as liquid bottom (10), light components, ethylene and residual isobutane as a vapor stream (12) and remaining isobutane exits as a liquid side stream (11). The distillation system comprises controller means such as ratio controller (not shown), for adapting the ratio of the bottom stream flow removed from the third distillation zone to the overhead stream flow introduced in the third distillation zone.