Integrating lignocellulosic bioethanol production with combined heat and power (CHP) production in polygeneration systems is considered an efficient and competitive way to produce a sustainable fuel for the transportation sector. This study assessed the energy economy of integrating lignocellulosic bioethanol production in the Danish CHP unit Avedøreværket 1. Numerical models of the plants were developed, and feasible integration solutions were identified and optimised using exergy analysis. Hour-wise production simulations were run over a reference year, and market prices and economic parameters from the literature were used to evaluate the production economy. A competitive energy cost limit for the bioethanol production was found to be 0.22 Euro/L. The optimised system produced bioethanol at a mean cost of 0.14 Euro/L during integrated operation and 1.22 Euro/L during separate operation. Maintenance shut-downs and periods of high power demand resulted in 3375 hours of separate operation over the year, giving an average bioethanol energy cost of 0.56 Euro/L. The results suggest that the polygeneration system cannot produce lignocellulosic bioethanol competitively under the given conditions, which questions the economic viability of the polygeneration system if operated in grids with periodically large power demands, for instance those caused by the operation of wind turbines and photovoltaic cells with a large capacity.