A process for preparing polyethylene from "feed ethylene" comprises: a) a hydrogenation stage in which "feed ethylene" containing impurities or secondary components such as acetylene and ethane is reacted with hydrogen to remove the acetylene by catalytic hydrogenation to form ethylene and part of the ethylene is converted into ethane, and b) a polymerization stage in which the ethylene leaving stage a) is reacted in the gas phase in a fluidized-bed reactor to form polyethylene, where the fluidizing gas used comprises, on entering the reactor, ethene and from 20 to 70% by volume of ethane, based on the total volume of the fluidizing gas, possibly together with further components, where, in a), ethylene is converted in a targeted manner into ethane in addition to the ethane already present in the "feed ethylene" so that the concentration specified in b) results. An apparatus for carrying out the process is also provided. In the process claimed, ethane is used to increase the heat capacity of the fluidizing gas used.