A novel approach for the numerical simulation of the development of crystallinity and birefringence on the spinline was proposed. The approach was based on the calculation of elastic recovery and crystalline and amorphous orientation functions without making any assumption about freezing. To model crystallization, the amorphous orientation function and the flow effect on the equilibrium melting temperature elevation due to the entropy reduction between the oriented and unoriented melts were incorporated. The crystalline orientation function was calculated from the frozen-in elastic recovery. The entropy change and frozen-in elastic recovery calculation were based on a nonlinear viscoelastic constitutive Eq. with the crystallinity and temperature dependent viscosity and relaxation time. The crystalline and amorphous contributions to the overall birefringence were obtained from the crystalline orientation function and the flow birefringence, respectively. The temperature, diameter density, and birefringence profile in both low and high-speed spun PET fibers were predicted and compared with the experimental data from literature. Theoretical predictions were found to be in agreement with the published experimental data.