This paper discusses theory and experiments on nonisothermal foam growth during foam sheet formation in an extrusion process. The extruded foam sheet expands and cools simultaneously when exposed to ambient temperature. A viscoelastic cell model in the Literature was modified to include heat transfer and gas loss effects during foam sheet formation. Experiments were conducted using a twin-screw extruder to study the effect of ambient temperature and initial sheet thickness on foam characteristics. The foam was made using low-density polyethylene with CFC-12 as the blowing agent. The experimental results are compared with theoretical predictions to check the validity of the model. The results reveal that heat transfer effects become important when sheet thickness decreases to the millimeter range. Agreement between theory and experiment is good when an appropriate boundary condition, to account for the gas loss, is included in the model.