The controlled pyrolysis of polyetherimide (PEI) and similar polymers can be used to obtain rigid and nanoporous carbonaceous materials, applied as carbon molecular sieve membranes for gas separation, among other purposes. Most of the knowledge regarding the control of their final properties according to the pyrolysis conditions, as reported in the literature, is empirical. In this work, we present results of a comprehensive analysis of reactive molecular dynamics simulations to investigate the dependence of the temperature, initial density and processing time on the morphological, compositional and structural features of carbonaceous materials formed upon PEI pyrolysis, as well as to identify the most common reaction mechanisms responsible for the transformations in the material. Our results show how the aforementioned variables affects significantly the structure and composition of the material, and emphasize the importance of controlling processing conditions toward optimized properties for practical applications. Our findings agree with experimental results from the literature, helping to understand them from a most fundamental point of view and improving the knowledge on how to control the underlying features of PEI-based pyrolyzed materials.