An increasing share of the weather-dependent RES generation in the power system leads to the growing importance of flexibility of conventional power plants, which usually were designed for base load operation. In the case of Combined Cycle Gas Turbine power plant (CCGT) one way of increasing its operation flexibility is integration with an Adiabatic Compressed Air Energy Storage system (ACAES). In this paper, we investigate techno-economic effects of CCGT-CEAS integration on operational flexibility. The key assumption of the CCGT-ACAES integration concept used in our model is the possibility of charging the compressed air storage with the use of electricity generated in the CCGT plant. In order to evaluate flexibility improvement, we optimize the operation of the CCGT-CEAS plant over a yearly horizon. Polish electricity day-ahead prices are used as a measure for remunerating flexibility. We use a mixed integer linear programming to find the optimal generation schedule. Solution methods are outlined. Computational simulations are performed in a realistic study case.