Postgres Installed in Linux (RHEL) and trying to access / connect using pgadmin (Windows)
First try to be more specific:


Which version of RHEL are you using?
Which version of Windows are you using?
How are these computers connected?


Are they in the same network segment?
Are there any firewalls in between



Next, try the following:


Check firewall configurations


Windows firewall configuration (or any Antivirus/Firewall software that may be blocking the pgAdmin)
SELinux configuration and/or any antivirus/firewall that may be preventing connections to the linux host
If the host is a 'hostname' then check that the network services is translating the name correctly. Try a ping to the IP and then ping to the hostname. Or issue a dnslookup.

In the linux system try netstat -a | grep LISTEN |grep "5432" to verify that there is a process actively LISTENING on port 5432


Hope this helps you to start solving your problem.