Using nonequilibrium molecular dynamics simulation, we have studied the response of a C100H202 model polyethylene melt to a step change from equilibrium to a constant, high shear rate flow. The transient shear stress exhibits pronounced overshoot at the strain value predicted by the reptation model, in striking similarity to melts of longer, entangled polymer governed by reptation motion. The Doi-Edwards theory is found to be applicable but only by taking into account the shear-rate-dependence of the terminal relaxation time (which is also consistent with the calculated shear-rate-dependence of the xx component of melt's self-diffusion tensor). We also analyze the molecular origins of stress overshoot behavior in short polymer chains by decomposing the shear stress and normal stress differences into the contributions from various molecular interactions and by examining the shear-induced ordering in the C100H202 melt.