In this work, a detailed nonisothermal model for a solid-state polymerization reactor of poly(ethylene terephthalate) (PET) is developed, discussed, and analyzed. The suggested model simulates the dynamic behavior of a continuous moving-bed reactor for the PET solid-state polymerization (SSP) process. The system of partial differential equations that arises is solved numerically to analyze the process startup or the process transients related to disturbances in the input variables and process parameters. The evolution of the process variables is described both inside the polymer and along the polymerization reactor. The results are compared with experimental observations available in the literature, highlighting significant insight regarding process mechanisms and possible alternative unit configurations and operating conditions. In the second part of this work (see the next paper in this issue), the results of this study are extended to the development of a reliable and effective control system.