Steam condensation in tubes exhibits obvious thermodynamic non-equilibrium characteristics, which leads to many problems, such as the decline of condenser efficiency and the freezing crack of condenser under low temperature. Limited by the tube size and the complexity of the condensation process, previous studies were mostly conducted with lumped parameter or one-dimensional model. As a result, the basic affecting mechanism of the problems existing in the application of condensers has not been revealed. This paper introduces developed by authors calculation method, which can quickly solve the coupled control equations involved in a three-dimensional (3D) steam condensation process. As the method was adopted to study the typical thermodynamic process of the steam condensation in an inclined tube, the 3D distributions of condensation heat transfer coefficient (HTC) and liquid film thickness in the whole inclined circular tube are obtained. Based on a comparative analysis together with the results of the referenced lumped method, it is found that the liquid film is the main source of the thermal resistance during a condensation process, and its variation along the circumferential and axial directions leads to the variations of HTC correspondingly. These fundamental mechanisms may explain the complex thermodynamic processes like freezing crack of condensers under low-temperature environment. The results obtained in this paper were verified by well-accepted 2D-distributed model of HTC and relevant experimental data with high accuracy.