The easiest way I know to do this is to combine them in Excel first and then export to your text file. First, you will want to remove any spaces in column A. You can do this in column A if you want or you can copy and paste this info into a new column, say column C. Use Ctrl+H to do this. In the first box (find), type a space only. Leave the second box (replace with) blank. Hit replace all. This will give you the result texasrestaurant from texas restaurant.
To append the domain, enter this formula in cell D1
=C1&"."&B1
Using the & is an easier way of doing CONCATENATE. You can use this to combine the contents of any two cells (number, text, etc...) into one cell. If you want to put any kind of text or special characters - in this case a period - between the two sets of data, place it in between double quotes (") and it will give you the desired output. In this case, texasrestaurant and com will give you a result of texasrestaurant.com
Copy column D and click on Sheet 2 (or another sheet) and use Paste Special to place the data. Select Values from the pop up window and select OK and then hit enter to paste. You can now export this sheet to your text file to get only the email addresses you want.
Alternatively, if all of the domains are .com domains, you can simplify this by using this formula:
=C1&".com" to simply append a .com to your space-removed restaurant name (from column C).
May seem complicated but is actually pretty easy to do. If you need additional tips on merging text, use the help menu in excel and do a search for Combine Text. There is plenty more info and some good examples there.