Abstract: The invention disclosed herein is a single-pass, horizontal heat exchanger, of the shell and tube type, which is designed for preheating of a viscous material, such as the reactor effluent of a polyethylene solution. The heat exchanger is made in three sections, each of which can be heated independently with a heating fluid, so that a temperature profile can be easily maintained along the entire length of the exchanger. Also, each section can be removed from the exchanger unit independently of the other sections, for cleaning, if the section becomes plugged.