In apparatus for polymerizing high-pressure ethylene to produce polyethylene, a tubular reactor is connected to a source of high pressure ethylene. An intensifier supplies an initiator solution to the tubular reactor. This solution includes one or more catalysts which promote the polymerization reaction. Within the tubular reactor, a peak temperature occurs in proximity of the point at which the catalysts are introduced. If the catalysts are introduced at more than one point, peak temperatures will result downstream of each of such points. For various reasons, the peak temperatures are displaced along the tubular reactor and a plurality of thermocouples are distributed at spaced positions in the tubular reactor to measure the temperatures at the positions at which the peak temperature might occur. These thermocouples generate signals which are fed into a gating arrangement which selects the peak signal which is compared in a comparator circuit with an adjustable or selectable signal representative of the peak temperature which is desired. A difference signal is generated which relates to the difference between the selected signal and the peak signal with which it is compared. This difference signal is converted by a transducer into a pressure signal which is fed to a control valve which controls the speed of operation of a pump feeding the catalysts into the reactor. If two or more pumps are employed for feeding the catalysts into different zones in the reactor, separate controls are provided for each of the pumps.