Geothermal fluids contain noncondensable gases (NCGs) at various amounts. The presence of NCGs in geothermal steam results with a dramatic decrease in net power output increasing condenser pressure and total auxiliary power consumption. Hence, NCGs should be withdrawn by a gas removal equipment to improve the performance of geothermal power plants (GPPs). The flashed-steam GPPs (single-flash, double-flash) are a relatively simple way to convert geothermal energy into electricity when the geothermal wells produce a mixture of steam and liquid. The primary aim of the Thesis is to model and develop a code to simulate flashed-steam GPPs to examine the thermodynamic and economical performance of NCG removal systems, which are major concerns at planning and basic design stages of GPPs. The model is validated comparing model output with Kizildere GPP output, classified as deterministic and static. The model is simulated to identify the effects of input variables which are NCG fraction, separator pressure, condenser pressure, wet bulb temperature, interest rate, tax rate, O&M cost ratio and electricity sales price. Among the variables, NCG fraction is the most significant parameter affecting thermodynamic performance and profitability of flashed-steam GPPs. The net power output and overall exergetic efficiency of singleflash GPP is decreased 0.4% for compressor system (CS), 2.2% for hybrid system (HS), 2.5% for reboiler system (RS) and 2.7% for steam jet ejector system (SJES) by 1% increase in NCG fraction. Based on thermodynamic and economical simulations, SJES, HS and CS can be recommended to be used for a NCG fraction range of 0-2%, 2-10% and >10%, respectively. Furthermore, thermodynamic performance of single-flash plants can be improved by adding second flash by 45.5-127.9%.