Polymerisation process in which polyethylene is produced in slurry in a polymerisation reactor in the presence of a Ziegler Natta catalyst and an activator, and a stream or slurry containing the polymer is withdrawn from the reactor and transferred to a flash tank operating at a pressure and temperature such that at least 50 mol % of the liquid or non-polymer component of the stream entering the flash tank or slurry is withdrawn from the flash tank as a vapour and at least 98 mol % of the vapour withdrawn from the flash tank is capable of being condensed at a temperature of between 15 and 50° C., without compression. A by-product suppressor, which reduces the amount of by-product formed per unit of polyethylene produced by at least 10%, compared with an identical polymerisation process where the by-product suppressor is not present, is used in the reactor. The molar ratio of the by-product suppressor added to the reactor to titanium added to the reactor is between 0.2 and 1.