Process for polymerizing olefins in the gas-phase

A gas-phase polymerization reactor for the gas-phase polymerization of olefins comprising:

• a polymerization zone having a recycle line for
• (a) withdrawing reaction gas from the reactor,
• (b) leading the reaction gas through a heat-exchanger for cooling and
• (c) feeding the reaction gas back to the reactor,
• wherein the recycle line has
• the heat-exchanger,
• a centrifugal compressor comprising variable guide vanes, and
• a butterfly valve,
• wherein the variable guide vanes are arranged upstream of the centrifugal compressor and the butterfly valve is arranged downstream of the centrifugal compressor.