Despite its complexity, reactive extrusion is continuously developing for the production of new and performing materials. Due to the strong coupling between flow, rheology and chemistry, the optimization of this process for a given reaction remains a difficult task. Moreover, the scale-up from laboratory to production scale is another crucial question, which cannot be solved by classical techniques. In this presentation, choosing the example of a transesterification reaction, we show how the use of numerical modeling may help to answer these complex questions by providing rapidly and without excessive cost realistic solutions.