Natural fuel burning in modern heat power plants with flue gas formation is accompanied with harmful environmental impact and potential opportunity of acid fallout. This can lead to local or even global catastrophe under adverse circumstances. High-rise structures combining a smokestack and a cooling tower are used for ecological natural fuel burning. The most important problem of these structures design is to study gas-dynamic behavior of flue gas mixing with warm ambient air flow during its movement along exhaust duct. Mathematical model and efficient numerical technique for problem solution have been developed in this work. The flowfields and the temperature and concentration distributions are calculated for various inlet conditions. Swirl flow ratio influence on flow nature has been studied. Obtained solutions have been compared with available experimental data and numerical researches. Calculation results can be used in search of optimum building structures.