In the present work, a numerical model is developed to analyze the riser-downcomer system of a natural circulation steam generator. The design and operation of the riser-downcomer system involve many complex issues such as multiphase flow inside the riser tubes, numerous possibilities of different flow regimes, and undesirable tube overheating due to the occurrence of critical heat flux (CHF). A separated flow model is developed to analyze steam-water two-phase flow inside the heated riser tubes. Further, the model is coupled with the implementation of the complex flow regime map and evaluation of wall temperature rise of the riser tubes. The present model is adequately validated with the existing experimental and numerical data as a direct problem. The model accuracy in predicting the tube dry-out and practical design is tested with the experimental data and real plant data, respectively. A typical 600 MW thermal power plant boiler is then investigated along with techno-economic efforts to find the possible design solutions of the boiler riser-downcomer circuit. The safe and unsafe zones of operations have been identified in the present study and, consequently, a range of feasible design solutions is provided in great detail. The diameters and thicknesses of the tubes used in the present analysis are in compliance with the ASME boiler code.