Isothermal experiments on gas-assisted displacement of viscoplastic Liquids in tubes show that a liquid coating remains on the tube wall. The thickness of this coating approaches 0.35 of the tube radius at high gas penetration rates, the asymptotic limit previously observed for Newtonian liquids. At low gas penetration rates, the viscoplastic coating is much thinner than its Newtonian counterpart. During the displacement process, the gas front moves faster than the liquid front and, prior to blowout, it rapidly accelerates as the amount of liquid downstream of the gas is depleted by the liquid coating. Based on these observations, a simple isothermal model is developed to describe the gas-liquid dynamics. This model provides an insight into the gas-assisted injection molding process in which the injection of molten plastic into a mold is assisted by a pressurized gas. In particular, the results show that the wall thickness around the hollow cores in gas-assisted parts is set during processing by the solid skin and a thick molten layer. The thickness of the molten layer depends on the gas penetration rate and the viscous behavior of the molten plastic. By coupling the isothermal model with a one-dimensional heat-transfer analysis, the gas penetration rate is shown to be several orders of magnitude higher than the plastic freezing rate, so that most of the heat transfer between the melt and the gas occurs after the mold is filled.