The wire-coating coextrusion process has been simulated numerically using the Lubrication Approximation Theory (LAT) and the Finite Element Method (FEM). The analysis includes both Newtonian and shear-thinning fluids under isothermal or nonisothermal conditions. Special Streamline-Upwind/Petrov-Galerkin schemes have been used for an accurate solution of the energy equation in highly convective flows. The determination of the unknown a priori interface and free coating surface was carried out through an iterative procedure that allows for no cross-flow along these surfaces. The analysis shows how an initial die design for high-speed wire-coating coextrusion can be modified to meet several criteria deemed necessary for better operation. These involve: (a) lack of recirculation everywhere inside the die, (b) the development of smooth stresses along the die wall, especially near the contact region of the two melts, and (c) the location of an interface that runs almost parallel to the wire. The improved die design that meets these criteria consists of moving the torpedo to a position of optimum “gum space” and also reducing the taper in the die region. Subsequent non-isothermal studies of two typical polymer resins, namely HDPE and PS, show that the initial melt and boundary temperatures do not influence so much the results and especially the interface location as do the individual flowrates (hence flowrate ratios) for a given total coating thickness. The analysis also provides a wealth of information regarding pressure and shear stress distributions, stream-line patterns and temperature fields, along with such overall quantities as pressure drops, wire tension, maximum temperature rise and maximum stresses.