A process for preparing a polyethylene in a multi-stage process is described. The process comprises pre-treating a Ziegler catalyst in a first stage in the presence of a 1-olefin/ethylene mixture or a 1-olefin to produce a LLDPE or VLDPE, which have the characteristics of a polymer prepared with a single-site catalyst, e.g. high levels of short-chain branching that are uniformly distributed. The contents of the first stage are then transferred to a second stage where an ethylene or an ethylene/1-olefin mixture is polymerized in the presence of the pre-treated catalyst to form a polyethylene with good processability.