Unixmen posted a guide about installing Install Nagios Enterprise XI on RHEL or CentOS

Nagios XI is the most powerful IT infrastructure monitoring solution on the market. Nagios XI extends on proven, enterprise-class Open Source components to deliver the best monitoring solution for today's demanding organizational requirements.
Designed for scalability and flexibility, XI is designed to make problematic IT monitoring tasks simple, while retaining the powerful attributes of its enterprise-class foundation blocks.

  Install Nagios Enterprise XI RHEL/Centos/SL