A process for polymerizing ethylene in the presence of a polymerization catalyst by copolymerizing ethylene with a comonomer selected from the group of C4-C10 alpha-olefins in three polymerization stages. The polymers produced in the three stages have different melt flow rates. The polymer composition produced by the process has good mechanical properties and can be used for making pipes. The process has a good productivity and provides a stable and economic operation.