A process for the removal of by-product polyethylene from a continuous flow, mixed, oligomerization reactor, the reactor comprising an agitator, said process comprising: a) providing input flows comprising ethylene, solvent, and an oligomerization catalyst system to said reactor; said oligomerization catalyst system comprising a source of active chromium, an activator and a diphosphine ligand defined by the formula (R 1)(R 2)-P 1 -bridge-P 2 (R 3)(R 4) wherein R 1, R 2, R 3 and R 4 are independently selected from the group consisting of hydrocarbyl and heterohydrocarbyl and said bridge is a moiety that is bonded to both phosphorus atoms; b) oligomerizing ethylene under continuous flow conditions within said reactor; and c) providing a discharge stream from said reactor comprising solvent, oligomer product and polyethylene by-product; characterized in that said process is conducted in the presence of a mass of reactor cleaning bodies, with the provisos that 1) substantially all of said cleaning bodies remain within said reactor during said process; 2) said mixed reactor is operated under mixing conditions that provide sufficient velocity to suspend said cleaning bodies within said reactor; and 3) said cleaning bodies have a particle size of from 2 millimeters to 2 centimeters; wherein the process further comprises using Correlation below to calculate and set an agitator speed sufficient to suspend the cleaning bodies within the reactor during the ethylene oligomerisation: Njs = S μ ρ 0.1 g ρ p − ρ ρ − 0.45 X 0.13 d p 0.2 D − 0.85 wherein: Njs is the just suspended speed in rad/s, S a coefficient specific to a particular agitation system, μ the liquid viscosity in Pa.s, ρ the liquid density in kg/m 3, g the gravitational constant (9.81 m/s 2), ρ p the cleaning body density in kg/m 3, X the dimensionless mass ratio of suspended solids to liquid x1000, d p the cleaning body characteristic diameter in m, and D the agitator characteristic diameter in m. a) providing input flows comprising ethylene, solvent, and an oligomerization catalyst system to said reactor; said oligomerization catalyst system comprising a source of active chromium, an activator and a diphosphine ligand defined by the formula (R 1)(R 2)-P 1 -bridge-P 2 (R 3)(R 4) wherein R 1, R 2, R 3 and R 4 are independently selected from the group consisting of hydrocarbyl and heterohydrocarbyl and said bridge is a moiety that is bonded to both phosphorus atoms; b) oligomerizing ethylene under continuous flow conditions within said reactor; and c) providing a discharge stream from said reactor comprising solvent, oligomer product and polyethylene by-product; characterized in that said process is conducted in the presence of a mass of reactor cleaning bodies, with the provisos that 1) substantially all of said cleaning bodies remain within said reactor during said process; 2) said mixed reactor is operated under mixing conditions that provide sufficient velocity to suspend said cleaning bodies within said reactor; and 3) said cleaning bodies have a particle size of from 2 millimeters to 2 centimeters; Njs is the just suspended speed in rad/s, S a coefficient specific to a particular agitation system, μ the liquid viscosity in Pa.s, ρ the liquid density in kg/m 3, g the gravitational constant (9.81 m/s 2), ρ p the cleaning body density in kg/m 3, X the dimensionless mass ratio of suspended solids to liquid x1000, d p the cleaning body characteristic diameter in m, and D the agitator characteristic diameter in m.