Reliable production of biofuels and specifically bioethanol has attracted a significant amount of research recently. Within this context, this study deals with dynamic simulation of bioethanol production processes and in particular aims at developing a mathematical model for describing simultaneous saccharification and co-fermentation (SSCF) of C6 and C5 sugars. The model is constructed by combining existing mathematical models for enzymatic hydrolysis and co-fermentation. An inhibition of ethanol on cellulose conversion is introduced in order to increase the reliability. The mathematical model for the SSCF is verified by comparing the model predictions with experimental data obtained from the ethanol production based on kraft paper mill sludge. When fitting the model to the data, only the yield coefficients for glucose and xylose metabolism were fine-tuned, which were found to be 0.43 g·g−1 (ethanol/glucose) and 0.35 g·g−1 (ethanol/xylose) respectively. These promising validation results encourage further model application to evaluate different process configurations for lignocellulosic bioethanol technology.