System dynamic models are typically impossible to solve analytically and even hard to correctly simulate. This complexity is also true in equilibrium and stability analysis. This research deals with analytical and numerical phase plane analysis of such models. Example models chosen have no analytical time trajectory solutions; they are nonlinear and computationally stiff. In the study, two example models are analyzed; a prey-predator model and a competition model. Numerical phase plane constructions of these systems are first carried out. Then, analytical phase trajectories are obtained by using LambertW functions, implemented by Maple software. Numerical work is carried out by Stella simulation software, and by numerical algorithms in MAPLE. Comparisons between analytical and numerical phase trajectories are made and it is observed that significant discrepancies may exist. The study shows that, although analytical time trajectories are typically impossible to obtain, analytical phase trajectories offer important potential for system dynamics