The scale-up of the synthesis of a very expensive key pharmaceutical intermediate requires a very accurate knowledge of the overall heat-transfer coefficient of the plant reactors. Those data allow the optimization of the safety, quality, and yield of the process. Starting from the accurate and reliable measurements determined by reaction calorimetry, the overall heat-transfer coefficient of the plant reactor can be evaluated, and the reaction performance can be simulated with a computer program. Here is reported an example of a reaction successfully scaled up to 1500-fold with excellent yield and high product quality.