Try this, Using the %{} make declaring a string over multiple lines easer and then you can also use direct #{} substitution for the variables which removes all the '+' and '"' characters.
@return_string += %{<tr bgcolor=white> <td>#{company.street_addr}</td> <td>#{company.city}</td> <td>#{company.state}</td> <td>#{company.pin}</td> <td width=10>#{company.branch}</td> <td width=10%>link_to_remote 'EDIT', :update => 'detail', :with => "company_id=#{company.id}", :url => { :action => 'edit_comp' }</td> </tr>}
David.