I am unable to create a variable in template.php that I later want to use in node--content-type.tpl.php
These are the steps I would take to resolve (re-trying after each step)


Clear theme registry cache - /admin/config/development/performance - Clear Cache
Use the more general function mytheme_preprocess_node($variables), clear cache, try again


If the more general function works, your issue is likely that you have incorrectly named the template, try:kpr($vars['theme_hook_suggestions']) if you have the Devel Module installed.  If not, you should. Otherwise you can print_r($vars['theme_hook_suggestions']).  Is your template included in there?