The video game industry is becoming increasingly important due to its revenues and growing capabilities. User eXperience (UX) is an important factor which contributes to the acceptance of a video game. The UX is usually assessed at the end of the development process, and for this reason it is difficult to ensure an adequate level of interactive experience between computer game and players. Cancelation of projects or even bankruptcy of a company can be caused by bad management of UX. In this paper, we propose the game experience management (GEM), a method to evaluate, manage, measure and track the UX from early stages of computer game development. In order to compare the proposal against a method comprised by conventional approaches, teams of master degree students were formed for developing six tower defense games for teaching basic multiplication operations; surveys were conducted to compare the UX of games. In this setting, we find that games developed with GEM significantly improve UX by increasing the puppetry and consequently reducing player frustration.