I'm running CentOS 6.4 in a virtual machine. In gnome-terminal,
$ echo DISPLAY
:0.0

With the above, all my graphical apps run with out issue. Been this way for years.

Recently I've begun working with a new technical application that sends this string to another application:

DISPLAY=myHostN ame:0.0

and the receiving application complains,

cannot connect to X server...