Fouling in a LDPE tubular polymerization reactor is caused by the polyethylene/ethylene mixture forming two phases inside the reactor. Some of the polymer-rich phase is deposited on the reactor's inside wall, which considerably reduces heat-transfer rates. At a given reactor pressure, the reactor inside wall temperature is the critical parameter in determining when fouling occurs and this is controlled by the coolant stream temperatures. Fouling upstream of a reacting zone increases the inlet temperature to that zone. This increase in temperature results in high hotspot temperatures, low fractional conversion, and broader polymer molecular weight distribution.