Currently, we are developing the IoT (Internet of Things) application server platform named Smart Environmental Monitoring and Analytical in Real-Time (SEMAR) for collecting, saving, analyzing, and visualizing data from various IoT devices. Unfortunately, the current implementation does not offer necessary functions for synchronizing and integrating data with independent IoT application systems. In this paper, we implement integration functions for data synchronizations, communications, and plug-in capabilities, using the MQTT protocol and REST API services in SEMAR. For evaluations, we integrated SEMAR with two IoT application systems, the air-conditioning guidance system and the fingerprint-based indoor localization system, using the implemented functions. The results show that packet losses and transmission delays at data communications can be reduced by the built-in MQTT server. Besides, the effectiveness of the implemented IoT application server platform is confirmed through qualitative evaluations of features with comparisons with existing state-of-the-art ones.