In this research, experimental adsorption measurements for a Dadas shale sample were conducted at various pressures (up to 13 790 kPa (2000 psia)) and temperatures (25, 50, and 75 degrees C) using pure methane (CH4). The effects of temperature and pressure on adsorption were observed. As pressure increases, CH4 adsorption increases. However, as temperature increases, CH4 adsorption decreases. Moreover, by using Langmuir isotherm and Ono-Kondo models, experimental adsorption results were evaluated and adsorption isotherms were constructed. Although Ono-Kondo and Langmuir isotherm models have similar fittings to the experimental adsorption data of Dadas shale, Ono-Kondo models have several advantages. Different from Langmuir isotherms, the volume of adsorbed fluid, absolute adsorption data, and surface area of Dadas shale sample were predicted by Ono-Kondo models. By comparing the results of the Ono-Kondo monolayer and three-layer models, the adsorption type of the Dadas shale sample was found as a monolayer (Type I). It was concluded that the Ono-Kondo monolayer model is capable of fitting adsorption isotherms, especially at high pressures for shale samples. For initial gas-in-place calculations, the equations derived using the Langmuir isotherm were modified by the Ono-Kondo monolayer model. For the first time, the Ono-Kondo monolayer model was used in the formula of initial gas-in-place calculation. This formula provides better adsorption data evaluation, initial gas-in-place amounts, and adsorbed gas ratios in shale gas reservoir conditions compared to the values obtained with the Langmuir isotherm. This method is suggested to be used in flow and gas production simulations of shale gas reservoirs.