A process for polymerizing or copolymerizing ethylenically unsaturated monomers at pressures in the range of from 110 MPa to 500 MPa in a production line comprising a continuously operated polymerization reactor, wherein at least one of a pre-heater or pre-cooler, a polymerization reactor or a post reactor cooler is composed of tubes of a length from 5 m to 25 m which are flanged together, either directly or via bends, and the flanges are covered by a chimney construction, and wherein air is conveyed through the chimney construction and the air exiting the chimney construction is monitored with respect to the hydrocarbons concentration.