According to the announcement from the Steam Community site, Portal is now officially supported by Linux systems. Portal has been a beta testing version for a while, but now it became stable.
Portal, Counter Strike and Team Fortress are the most popular Valve games, so it is good news to have Portal work nativelly on Linux.
First, you need to install Steam. See the following articles for more instructions.
After you have successfully installed the Steam client on your Linux system, click this link to start the Portal installation.