The structural and dynamical properties of polymer nanocomposites are investigated using stochastic molecular dynamics simulations. For spherical nanoparticles dispersed in a polymer matrix, the results indicate that the polymer-nanoparticle interaction strength and the overall system temperature are primarily responsible for the type of dispersed state (clustering and homogeneous dispersion) achieved. A systematic study probing temperature, polymerization, and polymer-nanoparticle and nanoparticle-nanoparticle interaction strengths has been performed. In this paper, however, we focus the discussion on the results for varying polymer-nanoparticle interaction strengths at different temperatures. By examining the structure and dynamics, we show that there are two kinds of "clustering transitions:" one due to thermodynamic and another due to the dynamical response of the system. From these results, a representative phase diagram is developed that captures the entire simulated space and allows the easy identification of the highly dispersed and the clustered states.