I would prefer writing a data class like this:
    [Serializable]
    [XmlRoot("acc")]
    public class Account
    {
        [XmlElement("Account")]
        public string Account { get; set; }
        [XmlElement("Partner")]
        public int Partner { get; set; }
        [XmlElement("CITY")]
        public string City { get; set; }
    }
and serialize / deserialize it with XmlSerializer.
        List<AccountClass> accounts = new List<AccountClass> 
        { 
            new AccountClass { Account = "test", Partner = 2144, City = "Munsbach" }, 
            new AccountClass { Account = "test", Partner = 2144, City = "Schuttrange" } 
        };
        XmlSerializer ser = new XmlSerializer(typeof(List<AccountClass>));
        using (FileStream fileStream = new FileStream("File.xml", FileMode.OpenOrCreate))
        {
            ser.Serialize(fileStream, accounts);
        }
Your file will look like this:
<?xml version="1.0"?>
<ArrayOfAccountClass xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <AccountClass>
    <Account>test</Account>
    <Partner>2144</Partner>
    <CITY>Munsbach</CITY>
  </AccountClass>
  <AccountClass>
    <Account>test</Account>
    <Partner>2144</Partner>
    <CITY>Schuttrange</CITY>
  </AccountClass>
</ArrayOfAccountClass>