I thought that natural gas was a mix of gasses including propane, butane, hydrogen, carbon dioxide...
Raw natural gas, unprocessed straight from the well, contains varying amounts of those items and others.
Before it is used for residential, commercial or industrial processes, it is cleaned up to mostly methane. That is what arrives at our homes. While some basic separation may happen at the upstream facilities, most of it happens at a Natural Gas Processing plant.
http://www.eia.gov/pub/oil_gas/natural_gas/analysis_publications/ngpipeline/process.html/
What is commonly called "pipeline quality" may contain some minimal amount of ethane and some others, it is mostly a BTU/volume restriction to prevent damage to burner tips and the like.
In the process of creating LNG, essentially all of these non methane components liquefy at temperature warmer than methane. They are separated out, and LNG is a very pure methane liquid, and resulting gas after vaporized.
Here is a typical requirement of Natural Gas to be accepted into a Pipeline Transmission System. Distribution Systems also have similar requirements.
http://www.northernnaturalgas.com/InfoPostings/GasQuality/Pages/Requirements.aspx