Nortel VPN client through Linux Proxy

I am having some problems with one of our customers accessing a Nortel VPN server on the Internet.

OK, here is the setup. There is a Linux server (Red Hat 6.1) that has 2 NICs. One has a public IP address and the other has a private IP address. It is doing NAT for clients on the LAN (one of which is the VPN client). It is using its only public IP to do NAT (Port Address Translation).

The VPN connection is not working.

Is there any thing special I need to do on the Linux server to allow this to happen?

Any help is highly appreciated.