This work presents a unified polymer reaction engineering methodology for the catalytic olefin polymerization process. The proposed modelling approach offers a modelling pathway from the polymerization recipe to production rate and polymer microstructure, and finally to rheological properties. Furthermore, this work introduces for the first time the constraint of the actual reaction performance of the polymerization catalyst in the inverse rheology and microstructural deconvolution problem, limiting the solution only to the most realistic potential molecular weight distributions (MWDs) that a specific catalyst can produce. This approach can be applied for both single‐ and multi‐site catalysts, providing not a potential MWD but the unique one that the selected catalyst can offer under given polymerization conditions. Depending on the available catalyst reaction performance insight, the constraint can vary and include from the number of active sites in use to the exact kinetic parameters of each site type. The potential of the proposed methodology is highlighted within a series of indicative examples, including forward, reverse engineering and deconvolution applications.