Precise measurements of pressure difference (Delta P) in capillary and slit flow of a commercial grade linear low-density polyethylene (LLDPE) were carried out using a multi-pass rheometer (MPR) fitted with stainless-steel die inserts. In addition, the experimental stress field for slit flow was determined from isochromatic retardation bands of the flow-induced birefringence (FIB) patterns by assuming the linear stress optical rule to be valid. The MPR results were compared with steady-state numerical predictions of the viscoelastic integral Wagner model, simulated using a finite element code, POLYFLOW. The effect of slip boundary conditions on the numerical predictions of Delta P and slit principal stress difference (PSD) was investigated; the comparison was made for the capillary Delta P data obtained at two temperatures (170 and 190 degrees C) and the slit results at 170 degrees C. For the lower stress data at 190 degrees C, a good match between experiment and simulation was obtained. At the lower temperature and higher stress it was necessary to introduce slip in order to match the results. Non-isothermal power law simulations suggest that shear heating effects cannot fully explain the low temperature extreme L/D data. Therefore, we tentatively conclude the necessity to introduce a wall high shear stress slip effect. For the slit geometry, the centreline principal stress difference birefringence tracking shows good agreement with the simulations. It was found that the simulated centreline \PSD\ was not very sensitive to the addition of wall slip. However, in order to obtain a good match with the overall pressure difference for the slit geometry, it was again necessary to introduce a slip component at the wall.